1. Field of the Invention
This invention relates to crystal growth and, in particular, hydrothermal crystal growth.
2. Art Background
The hydrothermal process is utilized for growing a wide variety of crystals. In this process typically a pressure vessel, e.g., a cylindrical pressure vessel, having a baffle or other means to produce a temperature gradient is partially filled at room temperature with an aqueous solution--a growth medium. Crystal nutrient material is introduced on one side of the baffle (or gradient), and seed crystals are introduced on the other. The pressure vessel is sealed and heating is initiated. The temperature gradient is adjusted to induce dissolution of the nutrient and growth on the seed. When the solubility of the nutrient increases with increasing temperature, such as in the growth of potassium titanyl phosphate, a suitable gradient is typically induced by heating the nutrient region directly and allowing the seed region to heat through conduction and/or convection. As the growth temperature is reached, the growth medium expands, fills the vessel, produces a significantly elevated internal pressure, and growth is initiated.
Generally, pressure must be limited by, in turn, limiting the degree of fill due to strictures imposed by processing conditions and vessel design. In particular, for the growth of crystals such as potassium titanyl phosphate, the vessel must be designed to have internal surfaces containing noble metals, e.g., gold or platinum, to prevent corrosion by the growth medium. Two configurations employing noble metal containers are utilized. In a first configuration, the vessel itself is lined with a noble metal material, and the seals of the vessel are formed by suitable mechanical interaction of the noble metal liner. Typically, this design is employed at pressures below 15,000 psi for 1 inch diameter vessels because at higher pressures, the noble metal seals are generally not adequate. For pressures above 15,000 psi, a second configuration is employed. In this configuration, a separate noble metal container 12, such as shown in FIG. 1, is present in the steel alloy pressure vessel 14. (In this configuration, 15 is the baffle, 16 is the seed, and 18 is the nutrient.) This container is filled with the growth medium and is surrounded by water to counterbalance the pressure generated during growth. In either vessel configuration, if nutrient region temperatures above 450 degress C. are required, the vessel itself must be an expensive, high temperature steel alloy rather than an easily machined, inexpensive, low carbon steel.
It is thus desirable to mitigate the growth temperature required for satisfactory growth of crystals such as potassium titanyl phosphate. In this manner, the possibility of employing a low carbon steel vessel is afforded. Lowering temperature, in turn, lowers the pressure at a given fill and also offers the possibility of employing the more desirable noble metal liner design. Indeed, compositions denominated mineralizers, e.g., potassium and phosphate containing glasses, are added to the growth medium to lower the required growth temperature. These mineralizers do have the effect of decreasing the pressure encountered at a given temperature while generally increasing the solubility of the nutrient in the growth medium.
However, to attain satisfactory crystal quality in an acceptable growth period, i.e., to attain growth rates of at least 0.04 to 0.1 mm/day/side, not only is an appropriate mineralizer necessary, but also the processing conditions must be suitably adjusted. Since the usable pressures and thus, in turn, the usable percentage fills are substantially limited by practical considerations, the growth temperature is the primary processing parameter adjusted to control the growth rate. Thus, for crystals such as potassium titanyl phosphate, where nutrient solubility increases with increasing temperature, it has been an overriding objective within pressure constraints to maintain relatively high growth temperatures, i.e., greater than 590 degrees C. in the nutrient region (despite the associated substantial inconveniences), to enhance, in turn, solubility and thus enhance growth rate. For example, as reported in "Nonlinear Optical Materials for Second Harmonic Generation (KTP)," by R. F. Belt et al, Avionics Laboratory, Air Force Wright Aeronautical Laboratories, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio, and in U.S. Pat. No. 3,949,323, issued Apr. 6, 1976, growth temperatures of at least 590 degrees C. in the nutrient region are required for the growth of potassium titanyl phosphate at an adequate rate.
As discussed, growth temperatures of 590 degrees C. require the use of a vessel having a separate noble metal container with a surrounding liquid to counteract pressure and requires a high temperature alloy steel. Unfortunately, this configuration has several undesirable features. For example, the presence of an additional container necessitates a smaller charge for a given vessel size and thus a smaller yield. It is difficult to maintain an equalized pressure around the container throughout the growth procedure. Thus, the container is often damaged, and concomitant costly repair is required. Additionally, the temperature gradient between nutrient region and seed region is less well controlled than in the liner configuration, and therefore it is more difficult to maintain crystal quality. The significant expense of high temperature alloy steel also substantially diminishes the desirability of the growth apparatus required for potassium titanyl phosphate.